A conventional boiling water reactor (BWR) includes a reactor pressure vessel containing a nuclear core for generating steam, with the pressure vessel being disposed inside a containment vessel capable of withstanding elevated pressures and temperatures for preventing significant radiation release in the event of an accident, such as a loss of coolant accident (LOCA). A shield building or other building structure encloses the containment vessel and provides a secondary barrier for further preventing release of radiation during the accident. The shield building used for more recent plants provides an annular plenum surrounding the containment vessel for the collection and filtration of radioactive fission product leakage from the containment vessel that may occur following an accident. The plenum annulus is normally kept at a negative pressure relative to atmospheric pressure so any leakage through the shield building or containment is into this plenum. Under accident conditions, the ventilation exhaust from this plenum is automatically diverted through a filtered standby gas treatment system (SGTS) before release to the environs.
The SGTS is a safety grade, active system typically using alternating electrical current power for driving redundant exhaust fans for drawing the contaminated air from the shield building annulus for filtering through conventional filter trains. The SGTS also includes associated heat removal fans, dampers, ducting and controls required to initiate action which will provide timely protection against the consequences of the release of airborne radioactive materials. The power supplies to this system allow uninterrupted operation during a loss of offsite power.
During an abnormal reactor condition such as the LOCA, a reactor scram is effected to stop power production from the reactor. However, steam generation continues at a reduced rate due to the core fission product decay heat. And, blowdown of steam from the reactor pressure vessel into the water pools within the containment vessel causes the containment vessel to be heated. Little heat is conducted through the containment vessel for the first several days following the accident since the walls of the containment vessel typically include concrete of about 2 meters thickness which has low thermal conductivity. Accordingly, several conventional additional systems are provided for suitably removing heat from within the containment vessel to prevent undesirable elevated temperatures and pressures therein.
In simplified boiling water reactor (SBWR) designs being designed, passive features are being considered for safety-related functions. A passive feature is one which does not rely on external power, such as alternating current (AC) electrical power, being provided for it to function. For example, following a LOCA, the residual heat generated from the reactor core is slowly conducted through the containment vessel which raises the temperature of the surrounding building regions. Heat generation also occurs from leakage of steam through containment pentrations as well as from radioactive decay of fission products postulated to be present in the steam. In one SBWR design, it is postulated that the resulting increase in temperatures due to this heat buildup may be acceptable for a few days following the accident without the need for active systems such as the SGTS, or the typical heating, ventilation, and air conditioning (HVAC) system using AC electrical power for cooling required portions of the reactor building. On a shorter time scale, heat generation also occurs from conventional equipment contained in typical reactor building rooms adjacent to the containment vessel which generate heat as they operate, such as various types of electrical motors.
Some commercial utilities, however, have expressed a desire that a SBWR have more than a few day capability to maintain safe plant operating conditions using solely passive features. Such features include obtaining acceptable cooling of the reactor plant as well as preventing unacceptable levels of leakage radiation from the plant. Some commercial utilities are requiring a capability as good as the best capability achievable from current BWRs using active safety systems. Therefore, additional capability using passive features is desired to extend the time frame to ensure equipment survival by limiting ambient air heatup, typically to a maximum of about 55.degree. to 65.degree. C., and to reduce the offsite radiation or dose leakage to as low as is reasonably achievable.